Tuned cavity assembly for harmonic generation of acoustic and electromagnetic waves of gigacycle frequencies



Feb. 27. 1968 P. H. CARR 3,371,264

TUNED CAVITY ASSEMBLY FOR HARMONIC GENERATION OF ACOUSTIC AND ELECTROMAGNETIC WAVES OF GIGACYCLE FREQUENCIES Filed Sept. l, 1965 waa wir( Pals! 4 l saaecf m S E l 19a/a /l f avez/M INVENT OR P Il. CII( BY fw'ww States of America as represented by the Secretary of the Air Force lFiled Sept. 1, 1965, Ser. No. 434,484 1 Claim. (Cl.I 321-69) ABSTRACT F THE DISCLSURE Apparatus for generating electromagnetic waves in the gigahertz frequency range from microwave energy at a lower fundamental frequency comprising a first resonant cavity tuned to the fundamental frequency and energized by the microwave energy, a second resonant cavity tuned to a harmonic of the fundamental frequency and coupled to an output circuit, and a rod of acoustic material having one end at a location of high electric field intensity in one cavity and the other end at a similar location in the other cavity. The acoustic material may be piezoelectric, such as X-cut quartz, or nonpiezoelectric, such as Z-cut quagtg.. In the latter case, however, f a piezoelectric .transducer must be attached to the rod end locateiie in the output cavity.

The invention described herein may be manufactured and used by or for the United States Government for govetnmental purposes without the payment to me of any royalty thereon.

The purpose of this invention is to generate acoustic and electromagnetic waves in the gigacycle frequency range through a process involving the generation of harmonics of acoustic waves produced in a medium by subjecting the medium to an electric field of fundamental frequency. The fundamental acoustic waves may be gen`= erated in the medium through operation of the piezoelec= tric effect in the case of a piezoelectric medium. Thus, it has been found that second and third harmonics may be- :produced in piezoelectric X-cut, AC-cut and BC-cut quartz, the second harmonic in X-cut quartz being par ticularly strong. However, second harmonic generation has also been observed in the case of acoustic waves produced in nonpiezoelectric Z-cut quartz and ruby as a result of the stress produced in the medium bythe fundamental frequency electric field. This latter acoustic wave generation process applies to all material media and therefore a technique for generating acoustic waves in any material is believed to have been demonstrated for the first time. In the past, acoustic waves have been generated by electromagnetic fields only in piezoelectric or magnetostriction media.

Briefly described, the apparatus for accomplishing the above comprises a pair of microwave resonant cavities, one tuned to the fundamental frequency and the other to the desired harmonic frequency. A rod of the acoustic material, quartz for example, has one end at a location of high electric field intensity in the fundamental frequency cavity and the other end at a location of high electric field intensity in the cavity tuned to the harmonic frequency. The first cavity is energized by radio fre= quency energy of the fundamental frequency at a rela= tively high power level. This results in acoustic waves of the fundamental and harmonic frequencies being gener ated in the rod, which travel down its length to the other end loctaed within the second cavity. If the rod is made of a piezoelectric material these waves produce electric fields of corresponding frequencies within the second cav- 70 ity due to the piezoelectric effect. If the rod is not made iiid Patented Feb. 2'?, 196@ of a piezoelectric material a small piezoelectric trans ducer is attached to its end to convert tliey acoustic energy into electrical energy. The second cavity'selects the har monic frequency to which it is tuned and is coupled to an output waveguide for supplying the desired electromag-1 netic output energy at that frequency.

The invention will be described in more detail with reference to the specific embodiment thereof shown in the accompanying drawings in which FIG. 1 is a cross-sectional view of the high frequency generator and FIG. 2 is a modification of FIG. l employed when the acoustic material is not piezoelectric. A

Referring to FIG. l, the assembly for lgenerating and detecting the acoustic harmonics comprises a resonant cavity 1 tuned to the fundamental frequency of, for ex= ample, 4.5 gc./s. (4.5 X109 c./s.). This microwave cavity is shown as of the cylindrical reentrant type having a reentrant stem 2 extending from one wall to a position of proximity to the other wall. Maximum etlectric field intensity in this type cavity exists in the spa'ce between the end .of the stem and the closely spaced wall. An opening the-wall opposite the end of the stem permits' the end of rod 3 lto be located in this region of high field intn sity. A second cavity 4, of the same type as cavity l, is made resonant to a harmonic of the fundamental fre= quency to which cavity 1 is tuned. This, for example, may be the second harmonic frequency or 9 gc./s. As in cavity 1, the other end of rod .3 passes through an opening in the wall of cavity 4 opposite the reentrant stem 5 so that its end is located in the high intensity electric field region between the end of the stem and the wall. One wall of cavity 4 may be a flexible diaphragm 6 to permit precise tuning of the cav= ity by screw 7. An output rectangular waveguide 8 is coupled to cavity 4 by an iris 9. Rod 3, for example, may be made of piezoelectric X-cut quartz and may have a diameter of about 3.00 mm. and a length of about l cm. In the operation of the above described apparatus, cavity 1 may be energized by microsecond pulses of 4.5 gc./s. energy at, for example, watts peak power. This energy is derived from source 10 which is coupled to the cavity by coaxial line 11 and loop 12. A high intensity electric field is developed at the upper end surface of rod 3 where the fundamental and harmonic acoustic waves are generated. The acoustic lwaves propagate along the rod, with a velocity in the case of X-cut quartz of 5.8)( cm./s., to the other end located in 'the 9 gc./s. cavity 4. Here the acoustic energy is converted into -high fre quency electric energy by the piezoelectric effect, the ere ergy at the second harmonic frequency of 9 gc./s. being selected by the critically tuned cavity 4. This energy passes out of cavity 4 through ywaveguide 8. Measure ments have shown that 100 watts of peak power into the 4.5 Igc./s.. cavity 1 produces 10 mw. of acoustic power at the fundamental frequency of 4.5 gc./s. -iid 0.1 mw. of acoustic power at the second harmonic frequency of 9 gc./s., the latter due to the nonlinearity of the generat-= ing process. With a detection efficiency in the 9 gc./s. cavity of -40 db, the detected electromagnetic power at 9 gc./s. was 0.01 nw.

It is also possible to excite cavity .l at 3 gc./s., in which case cavity 4, still tuned to 9 gc./s., selects the third har= monic. In piezoelectric X-cut quartz, the third harmonic acoustic power has been found to be 10J' of the fundamental acoustic power.

The same arrangement described above Imay be used when rod 3 is made of a nonpiezoelectric material such as -cut quartz or ruby. In this case the mechanical stress produced in the material by the electric field itself, or radiation pressure, rather than the stress produced by the repartent piezoelectric ei'ect, generates the acoustic wavesd Alsof, in this case, it is necessary that a piezoelectric transducer be attached to the end of the rod in the cavity Ituned to the harmonic in order to convert the acoustic energy into electrical energy for excitation of the cavity., This is illustrated in FIG. 2 where a piezoelectric transducer 13 which may be of the thin film cadmium sulphide type, is shown attached to the end of rod 3. Measurements in this instance, using Z-cut quartz or ruby for rod 3, showed acoustic power in. the microwatt region. and 10* nwa of electromagnetic power at 9 ^gc /s The entire assembly is preferably held at low tempera ture by submersion in liquid helium in container 14 to reduce the attenuation of the gigacycle acoustic waves below the room temperature valuet For optimum results the ends of rod 3 should be optically flat to one tenth of a wavelength of sodium light and parallel to within 6 seconds of arc. Any suitable mechanical means (not shown) for very slightly bending rod 3 to increase the para'llelism of the end faces has -been found to be an effective way to maximize the microwave output of the device.,

I claim:

1 Apparatus for generating electromagnetic waves at quency, said apparatus comprising: a rsi: resonant cavity tuned. Ito said fundamental. frequency; a second res` onant cavity tuned to a harmonic of' said fundamental frequency; a rod of an acoustic wave supporting nonpiezo` electric material having one end located in. a region of high electric field intensity in said first cavity and the other end attached to a piezoelectric transducer which is located in a region of high electric lield intensity in said second cavity; a source of high frequency energy of said yfundamental frequency coupled to said rst. cavity; and an output circuit coupled to said second cavity,

References Cited UNITED STATES PATENTS JOHN F., COUCH, Primary Examiner.,

a frequency harmonically related to 'a fundamental fre- 25 G- GOLDBERG, ASSI'SMH! Emmi-'18H 

